CHICAGO (AP)  Illinois Republican U.S. Sen. Mark Kirk, already facing an uphill battle for re-election, is entering the crucial final week before Election Day weakened by comments about his opponent that critics attacked as racist and “beyond reprehensible.”

Two organizations whose endorsements Kirk touted as indicators of his independence withdrew their backing over the weekend, issuing blistering rebukes. Both said they’re now supporting Democratic U.S. Rep. Tammy Duckworth.

Democrats are relying on winning Illinois as they look to regain control of the Senate. The party must pick up four or five seats to do so, depending on whether they retain control of the White House.

Kirk spent the weekend largely outside the public eye. Though he attended some community events, his campaign didn’t advise reporters or the public beforehand  a highly unusual move for any candidate in the waning days of a campaign, but one that allowed him to avoid rehashing the incident in the media. Duckworth had a full campaign schedule, including get-out-the-vote events with civil rights icon and U.S. Rep. John Lewis.

Kirk, who’s seeking his second term, apologized Friday for mocking Duckworth’s immigrant background and her family’s military service.

Duckworth, a veteran who lost both legs when the helicopter she was co-piloting was shot down in Iraq, said during a debate Thursday that her family has “served this nation in uniform going back to the Revolution.”

Kirk responded that he’d forgotten the congresswoman’s “parents came all the way from Thailand to serve George Washington.”

Duckworth’s mother is of Chinese descent, born in Thailand. The congresswoman, who was born in Bangkok, says her father first went to Southeast Asia while serving with the Marines in Vietnam.

Human Rights Campaign, a gay-rights group, and Americans for Responsible Solutions, a gun-control group led by former U.S. Rep. Gabby Giffords, both cited the remarks.

“Senator Kirk’s racist attack on Congresswoman Duckworth is unacceptable and represents a low point, even in this election season,” said Americans for Responsible Solutions Executive Director Peter Ambler.

Human Rights Campaign said it was the first time in the group’s 36-year history that it had reversed an endorsement.

“Attacking someone because of her race and ethnicity is inexcusable for anyone, but especially for a sitting U.S. Senator,” HRC President Chad Griffin said.

Kirk Campaign spokeswoman Eleni Demertzis said Monday Kirk is proud of his leadership in fighting discrimination and gun violence. Kirk has an “F” rating from the National Rifle Association and was the second GOP senator to support same-sex marriage.

“The HRC and ARS decisions are unfortunate because they were based on politics instead of reality and further exemplified just how uncomfortable they were in supporting a Republican who was a leader for their efforts,” Demertzis said.

Kirk had frequently pointed to the endorsements from the two groups, which typically back Democrats, as he works to persuade Illinois voters that he’s a moderate who can work with both parties.

Illinois typically elects Democrats to statewide office, and leans particularly Democratic in presidential election years  factors that led to Kirk being widely considered the GOP’s most vulnerable Senate incumbent.